The present invention relates in general to alcohol concentration sensing apparatus and methods used to determine the alcohol content of a person's breath and in particular, to a new and useful method and apparatus for determining the alcohol content of a test person's breath by showing a signal representing the concentration only when the rate of change of the signal has dropped below a preselected value and where the flow of breath is maintained above a preselected value.
The U.S. Pat. No. 4,090,078 concerns a method and an arrangement for determining the alcohol content in breathing air of a test person with an alcoholometer which is exposed to the exhaling air and which transmits the determined value of the alcohol concentration in the form of a signal S(t) when the variation of S(t) per unit of time as related to the height of the alcohol signal S(t) is below a given threshold value ##EQU1## and where the velocity of flow v of the exhaling air is above a given threshold value, and maintains this condition for a given time t without interruption. Methods and arrangements for measuring the alcohol content of the breathing air determine the real alcohol concentration when the portion of the exhaled air is tested for its alcohol value, that is, in equilibrium with the alcohol concentration of the blood in the alveoli of the lungs. The shuttle air from the oral and pharyingeal cavity and the mixed air from the alveolar air must therefore be measured separately.
According to the U.S. Pat. No. 4,090,078, the alveolar air portion is determined by monitoring the rise dS/dt of the alcohol concentration per unit time. This rise diminishes constantly with the alveolar air portion following the exhaled air from the oral and pharylingeal cavity until it stops completely when a concentration plateau is reached. When the rise, related to the instantaneous concentration, drops below the given threshold value w, only alveolar air is contained in the alcoholometer. Monitoring this rise in cooperative test persons would suffice to determine an alcohol concentration close to the value in the alveolar air. In practice, however, cooperation cannot always be expected. For this reason two additional conditions must be satisfied together with the monitoring of the rise before a measured value is indicated. The velocity of flow v determined by a flowmeter must be above a given value v.sub.min and must have been maintained for a given period of time.
A measuring instrument is used for the measurements with a short response time.